If you have low soil pH, should you lime?

It can take tonnes of lime per acre to move soil pH from 5 to 7, but improved fertilizer availability for all crops, better nodulation for pulse crops and alfalfa, and reduced risk from clubroot in canola can make the investment worthwhile

Soil pH is often highly variable within fields, as demonstrated in this map of a single central Alberta field. The range is from red (pH of 4-4.5) to dark lime green (pH of 7.5-8). That is why grid sampling is an important step in lime application. Photo:
Point Forward Solutions

“Finally.” That word got special emphasis when Doug Penney was asked about liming. “It has become a hot topic… finally.” Penney, a long-serving Alberta Agriculture fertility specialist and now semi-retired crop consultant, says many fields in Western Canada — especially in Alberta — probably would have benefited from lime a long time ago.

Fields most likely to benefit from lime have pH below 6. For pulses, pH below 6 will reduce rhizobium nodulation, and the Canola Encyclopedia says “on strongly acid soils with a pH of less than 5.5, canola yields are often reduced substantially.”

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The Canola Encyclopedia lists the factors that can limit canola plant growth on acid soils: toxicity of hydrogen ions, aluminum, iron, or manganese; deficiency of calcium, magnesium, potassium, phosphorus, boron, nitrogen or molybdenum; reduced organic matter breakdown and nutrient cycling by microflora; and reduced uptake by plant roots and inhibition of root growth. These same factors also inhibit yield potential for other crops.

Applications of lime — calcium carbonate (CaCO3) or hydrated lime (Ca(OH)2) — increase soil pH to help fix many of these problems. Lime is also proving to reduce clubroot gall formation in canola, a benefit that has elevated liming — “finally” — in the minds of farmers in the low-pH and clubroot hot zone in central Alberta.

“For everywhere else in the world with soil acidity, liming is a part of agriculture,” Penney says. He worked at the soil testing lab in Alberta for many years and when longtime Canadian farmers would get a test result with pH around 5.5 or 6, they wouldn’t think anything of it. “But when European farmers moved to Western Canada and looked at soil test results for low-pH fields, they’d be stunned. ‘Where the hell’s the lime?’ they’d ask.”

Why did those fields not show major productivity problems before clubroot?

“Aluminum toxicity is probably having a modest effect on yield in low pH soils,” Penney says. “But higher nitrogen rates, more inputs and better agronomy overall have offset that effect.”

However, he says, lime — though expensive due to large required volumes and trucking — could reduce the required rates for these other inputs. And, he adds, because nitrogen has an acidifying effect on soil, countering low pH with higher rates of nitrogen will make the situation steadily worse. “Soil pH in Alberta has dropped another half a unit over the past 20 years,” he says.

This map shows general soil pH across the Prairies for the top 0-5 cm. It is one of many soil maps available through the Science and Technology Branch of Agriculture and Agri-Food Canada.photo: AAFC

Experienced limers

Nathan Kosbau is an agronomist with CHS in central and southern Minnesota. Through his territory, soil pH ranges from 4.2 to 8.3, with quite a bit of variability even within fields. Liming has been a “thing” in Minnesota for the past 20 years, he says.

In Kosbau’s territory, 60 to 70 per cent of farms are corn-soy only. Most of the others have those two plus alfalfa, oats or sugar beets. For fields with low pH, the first target crop for liming would be alfalfa, he says. Next is corn to improve phosphorus availability.

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Cost-effective liming has to start with grid soil sampling to identify low pH areas, Kosbau says. The typical grid practice is one composite sample for every 2.5 acres. “Lime is pretty expensive when you look at it on a per-acre basis,” he says. “It doesn’t make sense to put lime in an area with higher pH.”

The common product used in Minnesota is AgLime, which flows well through broadcast applicators. Rates on those acres that need it most can be four to six tons per acre, but Minnesota farmers are motivated.

“They will spend more money to unlock the reasons why they’re not seeing more yield gain in corn and soybeans,” Kosbau says. Lime is one of those unlocking inputs, and applications will be repeated as needed: “pH is not static,” he says. “It is a moving target, not a destination.”

Tyler Waterhouse farms with his father and brother at Carberry, Man. They typically follow a three-year rotation of potato-cereal-oilseed. Their cereals are fall rye and wheat and their oilseeds are canola and soybeans.

Sandy areas around Carberry can have pH down around 4.5, which is very low for Manitoba — a province with mostly alkaline soils. The Waterhouses limed some of their low-pH areas for the first time around 2001, and liming continues to be part of their farm practice. They apply variable rates based on one-acre grid maps. Their lime source is the Brandon Water Treatment Plant, which uses lime to soften the water. The treatment plant provides the lime byproduct to farmers who will pay to truck it away.

Potatoes, a crop with high costs and high potential returns per acre, were the primary motivation for their lime investment and it provided “great results,” Waterhouse says. Wheat shows the biggest yield benefit and canola on limed land seems to hang on better in dry conditions, he adds.

In the time since they started liming, the Waterhouses have seen a big jump in average yields for their crops. But they’ve also improved a bunch of other crop management factors and weather conditions have also changed. Liming has likely been a positive factor, but how big a factor has it been? “We’re still learning about the long-term results of lime,” he says.

Tyler Waterhouse of Carberry, Man., works with Trevor Thornton of Crop Care Consulting on his soil pH management plan. Thornton provides these maps of a Carberry-area field before and after a long-term liming program. The map on the left is the original grid map from 2002. The one on the right is pH in 2016. The field received four lime treatments: three tons/acre, averaged across the field, in 2004; 2.5 tons/acre in 2007, two tons/acre in 2010 and 1.5 tons/acre in 2013.photo: Supplied

They don’t have clubroot, so that isn’t a motivator for them, yet. But they have clubroot in mind because the soil-borne disease is in Manitoba, seed potatoes can bring outside soil onto their farm, and potato production, with its mounding and harvesting, moves a lot of soil.

pH and clubroot

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Clubroot is driving the liming conversation in Alberta these days. Recent greenhouse trials show that clubroot galls will grow prolifically in solutions with pH of 6.5 but not at all in solutions with pH of 7.3. An Alberta study on two clubroot-infested fields with low pH shows that applying lime to increase pH to a neutral level will reduce clubroot and increase yield.

Dan Orchard, agronomy specialist for the Canola Council of Canada, says “the future of liming being part of clubroot control looks quite promising, either for patch management or larger areas.”

Targeting a pH of 7.3 could require tonnes of lime per acre. Orchard shares a quote from University of Alberta clubroot researcher Stephen Strelkov: “You can’t just sprinkle a little lime on the field and feel good about it.” This is more motivation for farmers to identify clubroot patches for intense management before the patches become too large.

“Understanding the right rate, right time, right product and right placement — the 4 Rs — of lime application seems to be instrumental in the success of liming,” Orchard says. While best practices to lime for clubroot remain a work in progress, Orchard reiterates Penney’s point that the benefits of liming in low pH Alberta soils go well beyond clubroot:

“Liming can improve yields through improved nutrient availability as well as nitrogen and phosphorus use efficiency. Wheat and barley will benefit if pH is 6 or higher. Peas will significantly benefit from bringing pH above 6 and closer to 7 is preferred. Alfalfa will be the big winner with raising the pH,” Orchard says. “There are no detrimental effects from raising the pH to 7, only benefits.”

Even if the best way to lime for clubroot management isn’t firmly nailed down, an investment in lime for low pH soils should help and — as a big bonus — provide the crop nutrition benefits that all other limers around the world are after. Finally.

Jay Whetter is communications manager with the Canola Council of Canada.

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